Long Limbs Helped Propel T. Rex Up the Dinosaur Food Chain

Surviving in the Cretaceous wasn’t a sprint, it was a marathon. And Tyrannosaurus rex was built to amble for hours, new research reveals. That attribute might have helped propel the carnivore to the top of the food chain, researchers suggest.

A study published earlier this month in PLOS One showed that some dinosaurs were particularly efficient walkers because of their long hind limbs. Thanks to their lanky legs, T. rex didn’t need to eat as much as its brethren, and could therefore get away with hunting less frequently, the team concluded.

To understand the biomechanics of long-extinct animals, scientists rely on fossilized bones and footprints. These records reveal information about a creature’s bone and muscle structure and its stride, all of which affect its ability to run. But identifying who is fleet of foot solely from imprints or a pile of bones is challenging.

In 1976, Robert McNeill Alexander, a British zoologist, proposed that a dinosaur’s maximum running speed depended on its stride length and hip height. But that idea has been revised over time.

“Once you get to big animals, limb length doesn’t really dictate speed,” said John Hutchinson, a biologist at the Royal Veterinary College in London who was not involved in the research.

In recent years, scientists have realized that long legs will only get you so far — body mass also plays a role. Elephants tower above thoroughbreds, but there’s a reason the Kentucky Derby is run with horses.

“Physics won’t let you go any faster” once you get too heavy, said Alexander Dececchi, a paleontologist at Mount Marty College in South Dakota. “Your muscles can’t get you to accelerate fast enough.”

To more accurately estimate dinosaur running speeds, Dr. Dececchi and his colleagues amassed measurements of hind limbs and published body mass estimates for 34 dinosaur specimens. For each of the specimens — ranging from a tiny Archaeopteryx, a birdlike creature weighing half a pound, to a 20,000-pound T. rex — Dr. Dececchi and his collaborators compared calculations of running speed.

The researchers determined that dinosaurs weighing less than a few hundred pounds were actually faster according to the calculations that used their body mass compared with the calculations that didn’t. In other words, smaller dinosaurs weren’t slowed by their heft.

But the situation changed for animals larger than about 2,000 pounds — those dinosaurs moved considerably slower, according to the equations that included their mass compared with those that just included stride length and hip height. For behemoths like Tyrannosaurus, that difference was significant: 18 miles per hour versus 45 miles per hour.

That schism left Dr. Dececchi and his colleagues wondering about the evolutionary advantage of lanky limbs for a massive dinosaur. “Their legs are longer than would help them for speed,” Dr. Dececchi said. Maybe those limbs allowed the animals to amble more efficiently, the team hypothesized. Dinosaurs like Tyrannosaurus walked about 10 miles per day, previous research has suggested.

Dr. Dececchi and his collaborators analyzed groups of dinosaurs with very similar masses but different leg lengths. For each animal, they estimated how much energy it would expend to move at a slow walk. They found that Tyrannosaurus used between 1 and 35 percent less energy than other related dinosaurs.

“That potentially gave them a big advantage,” said Eric Snively, a biologist at Oklahoma State University who was not involved in the research. “The amount of energy you use in a day will dictate how much food you have to eat.”

A Tyrannosaurus could get away with eating several hundred fewer pounds of meat each year than its brethren, Dr. Dececchi and his colleagues calculated. (That’s about one Ornithomimus, a beaked dinosaur that resembled a large ostrich.) Because the carnivores typically hunted in packs, that savings added up to tons less meat that needed to be hunted. Less frequent hunting no doubt helped preserve the animals’ health.